This invention relates to a technique for locating a high-resistance underground utility conveyance via electromagnetic signaling.
Many utilities, such as ATandT, bury their pipes and cables (xe2x80x9cutility conveyancesxe2x80x9d) underground both for reasons of safety and esthetics. Underground burial often provides protection to utility conveyances against weather and other sources of potential damage. Utilities that undertake burial of their conveyances usually make extensive efforts to plot the location of each buried conveyance on a map to facilitate its location in case of repair or replacement. While a map will indicate the general location of a buried conveyance, more precise location information often becomes necessary, particularly in urban environments. For that reason, most utilities that bury their conveyances underground typically rely on electromagnetic signaling techniques to precisely locate such conveyances.
U.S. Pat. No. 5,644,237, issued in the names of Hossein Eslambolchi and John Huffman, on Jul. 1, 1997, and assigned to ATandT, (incorporated by reference herein) discusses the principle of electromagnetic signaling for locating a buried utility conveyance. To locate a buried conveyance, a locating tone (signal) is applied to a metallic part of the conveyance. In the case of an optical fiber cable, the metallic part comprises either a metallic sheath or a copper trace wire within the cable. Using a signal detector of a type well known in the art, a technician detects the locating tone radiated above ground to precisely locate the conveyance.
Generally, the greater the amplitude of the locating tone applied to the metallic part of the conveyance, the greater the amplitude of the locating signal radiated above ground and hence, the more likely that a technician can detect the signal. The resistance of the metallic part of the conveyance typically limits the amplitude of the locating tone that can be applied. Optical fiber cables that employ a copper tracing wire can carry a higher amplitude locating tone as compared to optical fiber cables employing a high-resistance carbon-steel metallic sheath. Consequently, locating buried utility conveyances that have a high resistance metallic part has proven problematic by virtue of the reduced amplitude of the locating tone radiated above the ground.
Thus, there is a need for a technique for overcoming the aforementioned disadvantages associated with locating such high-resistance underground utility conveyances.
Briefly, the present invention provides a method for locating an underground utility conveyance having a metallic part that runs along its length. In accordance with the method, a locating tone is applied to a first end of the metallic part of the conveyance while the second end of the metallic part is coupled to ground. The metallic part of the conveyance is selectively shunted to ground at a point lying between the first and second ends, thus causing only that portion of the metallic part lying between the point selectively grounded and the first end to radiate a locating signal. Selectively shunting the metallic part of the conveyance to ground at a point between the two ends serves to reduce the effective length, and hence the resistance of the metallic part, thereby increasing the strength of the locating signal radiated above ground, making detection of the locating signal, and location of the conveyance more reliable.